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Carrera VDO Speedo SUCKS.

OK, so I've owned 5 Carreras manufactured before 1989, all with the VDO speedometer, and 4 of them needed to be repaired because that POS gear on the Odometer Breaks. This time, I did the repair myself, ordered the gear on the Internet (Wonderful Experience with that! http://www.odometergears.com/ ) Installed the gear. Went for a test drive, and the odometer works, but the speedo is pretty messed up- Was trying to calibrate it with a GPS- Wasn't working very well, so I build a test circuit to provide the pulses to the Speedo so I could calibrate it and set the needle while on the work bench.

Based on 245-45R16 wheels, and 8 pulses per wheel revolution I get a conversion factor or 0.5507 x Hz = MPH.

(Also based on 5280 feet/mile, 25.4 mm/inch, 12 inches/foot and pi=3.1415926, and 3600 seconds/hour.)

I'd write out the formula, but it looks to sloppy anyway.

So great. Put a 100Hz signal into the speedo input, and it should read about 55mph. This was all going pretty good, and I was getting ready to set the needle, when I did a test at 200Hz (About 110 mph, and then I thought I'd give it 300 Hz (165 MPH) just to check the full range. That's when the thing stops working- That is the odometer part still works- The stepper is happily stepping at 165 MPH, BUT the frigging circuit that drives the speedo gage decided to give up. No more needle deflection. Anymore.

Heavy Frigging Sigh.

I fed the speedo input with a frequency generator, set at 0-7 Volt output, through a 500 ohm resistor. The speedo biases the sensor wire to the transmission at about 8 Volts, and the signal is grounded by the transmission. I figured a 500 ohm resistor was just for safety.

Still the effing thing decided to stop working.

No more voltage is being sent to the deflection part of the speedo.

ARGHHHHHHHHHHHHHH!
I HATE VDO. I EFFING HATE VDO. I HATE VDO.

Here's a picture of the crap set up in my living room. Long story why its there, and not, where, well it should be.... And a picture of the VDO IC that is f*cked.

Anyone know a source for the French IC "ITT UAF 2115" on the circuit? The part of this chip that provides the excitation for the needle deflection is toast.


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2001 CLK55 AMG, 1987 911 Turbo Look, 1997 Viper GTS.
Old 10-24-2005, 11:35 PM
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US Bid should be able to source that part for you. I'm sure I don't need to explain ESD ... right? Your living room with carpet, electronics and wires everywhere is an electrostatic soup.
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Old 10-25-2005, 12:10 AM
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Paul,

Unfortunately, the proper input to the VDO electronic speedo is a set of 'dry' reed relay contacts ... NOT a Voltage source! This is documented in the factory service manual, and any instrument portion of the schematic since 1975 shows the contacts and input to the speedo.

The LM2917 can be configured on a new board to replace the board you took out ...

$1.49 LM2917 IC does tachometer, MFI Speed Switch, rev limiter, or shift light ...
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Old 10-25-2005, 12:10 AM
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Hey Guys, thanks for the advice. I've got my frequency generator set up so it is just sinking current- It is behaving like a reed switch to ground through a 500 ohm resistor.

Concerning ESD-- Yeah Yeah. I just shut down my RF radio modem Company, and well life is chaos right now. I'm used to a real lab, with ESD tables, and straps and and and... I live in Texas and it is pretty humid. Plus this isn't CMOS... It's old big ass PNP and NPN transistor technology in an 1987 automotive circuit... I think ESD is probably the last of my problems. :-)

ANYWAY! I found the chip info with a google search, and started debugging the board. I FOUND THE PROBLEM: The 100uF 16V Cap that was in parallel with the speedo gage was shorted internally- Measured 0.4 ohms, either way with a probe. (You can see it in the photo above-- The big black cylinder) I removed it from the circuit, and now the gage functions- With a little bit of a nervous quiver.

I'll install a new cap, and everything should be fine... Then I get to go back to calibrating the needle movement based on my 0.5507 conversion constant.

Here's the link to the ITT data sheet for the UAF2115
http://users.tpg.com.au/cobra30/images/R31/uaf2115.pdf
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Old 10-25-2005, 01:45 AM
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I tweaked my speedo reading by adjusting an inner pot. I also posted the info on a speedo thread.
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Old 10-25-2005, 05:17 AM
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Maybe you should start a business fixing VDO electronic speedos?

How much to do mine?
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Old 10-25-2005, 07:29 AM
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Getting away from the electronic side of the argument, which is Greek to me:
Quote:
Based on 245-45R16 wheels
This is a pretty important piece of the equation here. I calibrated my speedo also when I went from the 85 mph in my SC to the 150 mph from a 77. I measured the rolling radius of the wheel as the car sat on the ground, and calculated from that what the speed should be at a given series of engine RPM's using the final drive ratio and a particular gear ratio. I then pulled the speedo out, took off the bezel and reconnected it to the car, but hanging loose, jacked up the car and removed the rear wheels, fired it up and spun the engine to my calculated RPM's and observed indicated speed and adjusted using the pot earlier mentioned.

This of course assumes the tach is accurate, but if you're not an electronics guy, what else are you going to do?

Now my question for you and Warren. The speedo seems to work fine and is accurate when the ambient temp is cold. When the weather is hot however, like when the interior is very warm from sitting in the sun, the speedo behaves erraticly, and the needle will read very high and\or bounce around considerably. I did not observe this with the 85 mph speedo, so I'm fairly certain it's a problem with the 150. Any ideas or theories on what internal component could be flaky to cause this? Thanks,

ianc
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Old 10-25-2005, 10:34 AM
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Ian,

Actually, the same component [input filter cap] Paul found bad ... is well known for failing in all of the VDO speedos [Volvos, BMW, Mercedes, Audis, Ford, Nissan] using the UAF2115 chip. It is a bit marginal, at best, with the 16 Volt rating, so I would recommend replacing it with a 100 uF 50 Volt cap ... like this one:

http://www.jameco.com/webapp/wcs/stores/servlet/ProductDisplay?langId=-1&storeId=10001&catalogId=10001&productId=29963&pa=29963PS

It would seem that failing alternators that are boiling batteries with 16+ Volts output are also frying that marginally-rated capacitor ... and the indication of a pending failure is a jumping, or erratic speedometer needle!
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Last edited by Early_S_Man; 10-25-2005 at 04:46 PM..
Old 10-25-2005, 11:37 AM
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If your living room looks like that your either single or have a VERY understanding wife/girlfriend.
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Old 10-25-2005, 01:25 PM
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Quote:
Originally posted by bfrenz
If your living room looks like that your either single or have a VERY understanding wife/girlfriend.
Very keen observation- LOL... Two un-undertanding X-wives, and my pseudo girlfriend just moved to Houston-- while I continue to live in Dallas.... They are Not leaving me becuase I don't drive a cool enough car. That's for sure.
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Old 10-25-2005, 08:41 PM
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Ianc,

I calibrated a tachometer from a 1983 SC. The gage was not linear through the entire range, and you could adjust the needles deflection by bending aluminum tabs that effected the strength of the electro magnetic field that caused the needle to deflect... So If you are using the tach as the source for the speed, well then as you are aware, errors in the tach will then be present in your speed. Interestingly enough, other then setting the ZERO reference point for the needle on my 1987 VDO speedo, there was no other adjustment that could be made- No potentiometer on the circuit board.

I can definitely see that extreme temperature would effect the reading in an older speedo, especially one with a failing component. I can't offer any advice on which component this may be... But I will stand by statement that older electronic VDO gages really suck! :-)
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Old 10-25-2005, 08:49 PM
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Hey Warren and ewave,

Thanks for the info on the speedo. I posted a thread on this some time ago, but the best answer I could get was that it had a poor ground, which didn't seem likely to me.

Warren's answer gives me a direction to turn to, but lately I've been thinking about replacing the entire car as a means of fixing the problem!

ianc
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Old 10-25-2005, 09:23 PM
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Quote:
Originally posted by ewave

on my 1987 VDO speedo, there was no other adjustment that could be made- No potentiometer on the circuit board.
for the archives.. my stock '77 speedo has a pot.
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Old 10-25-2005, 11:36 PM
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I recently replaced the odometer gear that runs off the worm gear in my '87 930 speedometer and I remember seeing a little circut board potentiometer on the board.
Old 03-16-2009, 07:58 PM
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